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LTM8001

8001f

For more information www.linear.com/8001

and damaging the part. If the input supply is poorly con-

trolled or the user will be plugging the LTM8001 into an

energized supply, the input network should be designed

to prevent this overshoot. This can be accomplished by

installing a small resistor in series to VIN0, but the most

popular method of controlling input voltage overshoot is

to add an electrolytic bulk capacitor to the VIN0 net. This

capacitor’s relatively high equivalent series resistance

damps the circuit and eliminates the voltage overshoot.

The extra capacitor improves low frequency ripple filtering

and can slightly improve the performance of the circuit,

though it may be physically large.

Shorted Input Protection

Care needs to be taken in systems where the VOUT0 out-

put will be held high when the input to the LTM8001 is

absent. If the VIN0 is allowed to float and the RUN pin is

held high (either by a logic signal or because it is tied to

VIN0), then the LTM8001’s internal circuitry will pull its

quiescent current through its internal power switch. This

is fine if your system can tolerate this state. If the RUN pin

is pulled low, the input current will drop to essentially zero.

However, if the VIN0 is grounded while the VOUT0 output is

held high, then parasitic diodes inside the LTM8001 can

pull large currents from the output through the VIN0 pin.

Figure 7 shows a circuit that will run only when the input

voltage is present and that protects against a shorted or

reversed input.

Charging Applications

The LTM8001’s internal switching step-down regula-

tor’s CVCC operation makes it well suited for battery or

supercapacitor charging applications. A schematic of the

LTM8001 charging a supercapacitor and then distribut-

ing power to various loads through the onboard LDOs is

shown in the Typical Applications section. In this applica-

tion, the supercapacitor is charged through the step-down

switching regulator and not the LDOs. Each LDO is rated

for positive and differential voltages between its input

and output, but may experience a negative voltage during

start-up or turn-off transients if its output is connected to

a battery, supercapacitor or energized load. Avoid using

the LTM8001 in applications where the internal LDOs can

experience a negative voltage.

Thermal Considerations

The LTM8001 output current may need to be derated if it

is required to operate in a high ambient temperature. The

amount of current derating is dependent upon the input

voltage, output power and ambient temperature. The

temperature rise curves given in the Typical Performance

Characteristicssectioncanbeusedasaguide.Thesecurves

were generated by the LTM8001 mounted to a 59cm2

4-layer FR4 printed circuit board. Boards of other sizes

and layer count can exhibit different thermal behavior, so

it is incumbent upon the user to verify proper operation

over the intended system’s line, load and environmental

operating conditions.

Forincreasedaccuracyandfidelitytotheactualapplication,

manydesignersusefiniteelementanalysis(FEA)topredict

thermal performance. To that end, the Pin Configuration

of this data sheet typically gives four thermal coefficients:

θJA: Thermal resistance from junction to ambient

θJCbottom: Thermal resistance from junction to the bottom

of the product case

APPLICATIONS INFORMATION

VIN

RUN

VOUT0

GND

8001 F07

LTM8001

VIN

VOUT

Figure 7. The Input Diode Prevents a Shorted Input from

Discharging a Backup Battery Tied to the Output. It Also

Protects the Circuit from a Reversed Input. The LTM8001

Runs Only When the Input is Present

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